As communications networks become more complex, the task of network management becomes increasingly difficult. An important aspect of a network management system, or NMS, relates to determining and maintaining an accurate record of the topology or connectivity of the network. In optical WDM communications networks this may involve knowledge of not only connections of optical fibers among nodes of the network, but also connections of optical fibers within the nodes, allocation of wavelengths to respective optical fibers, and arrangements and sequences of multiplexers and demultiplexers, or optical band filters, within the nodes.
While it is conceivable to provide and maintain manually a record of the topology of a network, this becomes increasingly impractical as the network becomes more complex, and has other disadvantages such as being subject to errors and being slow and inconvenient to update and respond to changes.
In communications networks it has been proposed to provide automatic discovery of the topology of the network. For example, such proposals are disclosed for ATM networks in Suzuki U.S. Pat. No. 5,796,736 issued Aug. 18, 1998 entitled “ATM Network Topology Auto Discovery Method” and in Chatwani et al. U.S. Pat. No. 5,729,685 issued Mar. 17, 1998 and entitled “Apparatus For Determining The Topology Of An ATM Network Or The Like Via Communication Of Topology Information Between A Central Manager And Switches In The Network Over A Virtual Service Path”.
As disclosed in the latter patent, ATM cells include so-called link advertisement messages (LAMs) each of which identifies an originating switch and port number and is forwarded by a receiving or neighbour ATM switch to a network manager. The network manager thereby develops information profiling the topology of the network. Matching of LAM pairs is carried out by the network manager to confirm bidirectional NNI (Network-Network Interface) links.
While such a known arrangement may be effective for discovering neighbours in an ATM network, it requires handling of the LAMs specifically in each ATM switch and reduces bandwidth of the network for data traffic. In addition, such a known arrangement is not effective for determining the topology of an optical communications network, in which for example an optical fiber path between two nodes A and B may pass through an intermediate node C. The ATM cell or packet level which would be determined by such a known arrangement would indicate that the nodes A and B are coupled together and would not show the node C, whereas the actual topology in this case is that the node A is coupled to the node C, and the node C is coupled to the node B.
Also, Wood U.S. Pat. No. 6,108,702 issued Aug. 22, 2000 and entitled “Method And Apparatus For Determining Accurate Topology Features of A Network” discloses a system for monitoring packet traffic in a network to determine topology features using logical groupings of ports and/or devices. Orr et al. U.S. Pat. No. 5,727,157 issued Mar. 10, 1998 and entitled “Apparatus And Method For Determining A Computer Network Topology” discloses determining the topology of a computer network including data-relay devices based on a comparison of source addresses heard by the various data-relay devices.
In an optical network, Fee U.S. Pat. No. 6,108,113 issued Aug. 22, 2000 and entitled “Method And System For Transporting Ancillary Network Data” discloses superimposing a sub-carrier modulation signal at a relatively low frequency (e.g. 1 MHz), containing ancillary network data, on an optical carrier of a high bit rate (e.g. 1 to 10 GHz) data signal. This patent discloses that the ancillary network data can include any of numerous data types identifying any of numerous network elements, and can be used for any of numerous network management purposes one of which is listed as “Probing Network Topology”, but no further information in these respects is disclosed.
Fatehi et al. U.S. Pat. No. 5,892,606 issued Apr. 6, 1999 and entitled “Maintenance Of Optical Networks” discloses an apparatus for adding a dither signal to an optical carrier modulated with an information signal, to provide a method for monitoring and tracking end-to-end signal routing in multi-wavelength optical networks. This patent discloses that the monitoring can take place at any point in the network. This patent is not concerned with topology discovery.
A need exists for an improved method of topology discovery which is particularly applicable to optical WDM networks.